1. Field of the Invention
The present invention generally relates to an optical head in an optical disc player for optically recording and/or reproducing information on a disc-shaped recording medium and, more particularly, to a lens drive device used in the optical head.
2. Description of the Prior Art
The lens drive device currently used in the optical head in an optical disc player generally comprises an objective lens assembly; a guide shaft having a longitudinal axis extending parallel to the optical axis of the objective lens assembly, which optical axis extends generally perpendicular to the disc-shaped recording medium; a lens holder including a sliding bearing having the objective lens assembly mounted thereon, whereby, when a cylindrical inner surface of the lens holder having a center axis generally parallel to the optical axis of the objective lens assembly undergoes sliding and rotary motion relative to the guide shaft, the objective lens assembly can be moved in a tracking direction generally parallel to the disc-shaped recording medium and also in a focusing direction generally parallel to the optical axis; a base to which one end of the guide shaft is fixed; four tracking coils mounted on the lens holder in symmetrical relationship relative to the guide shaft, each of said tracking coils having a coil winding axis extending in a direction generally perpendicular to the longitudinal axis of the guide shaft; a focusing coil mounted on the lens holder and having a coil winding axis extending in a direction conforming to the direction of the longitudinal axis of the guide shaft; two focusing magnets mounted on the base so as to confront the opposite side faces of the focusing coil and disposed generally in symmetrical relationship relative to the guide shaft, each of said focusing magnets having a direction of magnetization generally perpendicular to the coil winding axis of the focusing coil; and four tracking magnets mounted on the base so as to confront respective side faces of the tracking coils.
The prior art lens drive device of the above-described construction generally performs the following operation.
The drive of the objective lens assembly in the focusing direction takes place when the lens holder undergoes a sliding motion along the guide shaft, secured at one end of the base, under the influence of an electromagnetic force which is obtained when the focusing coil secured to the lens holder carrying the objective lens assembly mounted thereon traverses the magnetic flux generated by the focusing magnet secured to the base.
The drive of the objective lens assembly in the tracking direction takes place when the lens holder undergoes a rotary motion along the guide shaft secured at one end to the base under the influence of an electromagnetic force which is obtained when the tracking coils secured to the lens holder carrying the objective lens assembly mounted thereon traverses the magnetic fluxes generated by the tracking magnets secured to the base.
In the prior art lens drive device referred to above, however, because the focusing coil and the tracking coil are both formed on the lens holder, the movable mechanism comprising the objective lens assembly, the lens holder and the focusing and tracking coils tends to be complicated in shape and, therefore, the distribution of the weight of the movable mechanism as a whole is dispersed making it difficult to improve the rigidity. Therefore, when the lens drive device is to be driven, it is difficult to improve the response characteristic of a displacement frequency in each of the tracking and focusing directions and also to reduce the distance between the axis of rotation and the objective lens assembly because of the axis of rotation of the lens holder being occupied by the guide shaft, thus making it difficult to provide the lens drive device in a compact size.